The Median Tectonic Line(MTL)is the largest active fault system on the Japan Islands and underlies many densely populated areas.To understand the seismotectonics and to reduce seismic hazard in the MTL region,we determined a detailed 3-D crustal structure under the region using a large number of arrival times of the first P-waves and Moho-reflected waves(PmP).Results of detailed resolution tests show that the addition of PmP data can significantly improve the resolution of the lower crustal structure and the entire crustal structure can be imaged better than that by using P-wave data alone.The obtained The Median Tectonic Line (MTL) is the largest active fault system on the Japan Islands and underlies many densely populated areas. Know the seismotectonics and to reduce seismic hazard in the MTL region, we determined a detailed 3-D crustal structure under the region using a large number of arrival times of the first P-waves and Moho-reflected waves (PmP). Results of detailed resolution tests show that the addition of PmP data can significantly improve the resolution of the lower crustal structure and the entire crustal structure can be imaged better than that by using P-wave data alone. The